The invention relates to a band-pass amplifier having a differential stage comprising two transistors fed from a joint current source.
The invention is concerned in particular with an elementary differential amplifier stage for intermediate and high frequency amplifiers which is used particularly in bipolar integrated circuits. The invention represents a contribution to overcoming two fundamental difficulties with bipolar integrated high-frequency amplifiers, namely the unwelcome direct current amplification and the unwelcome wideband nature of such amplifiers.
A known solution to the above problems is indicated in the Dutch patent 8003197. In the known circuit a differential stage comprising a power source and a first and second bipolar transistor is combined with a cross-coupled circuit comprising a third and fourth bipolar transistor of identical conductivity type. The collector of the first transistor is connected to the emitter of the third transistor, and the collector of the second transistor is connected to the emitter of the fourth transistor, the connection points doubling as the outputs of the circuit. The collectors of the third and fourth transistors are connected via a first and a second resistor to a supply voltage. In addition, the collector of the third transistor is connected via a third resistor to the base of the fourth transistor, and the collector of the fourth transistor via a fourth resistor to the base of the third transistor. The first and the second resistors are so dimensioned that the D.C. voltage amplification of the circuit is approximately zero. The third and the fourth resistors are used to set the required resonance frequency.
While this circuit has favourable properties in the high frequency range, e.g. in intermediate frequency amplifiers for TV receivers, its use in intermediate frequency amplifiers for radio is more difficult. The above circuit, in that case provides relatively high values for the third and fourth resistors, to the detriment of operating point stability.